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0 Department of Biology, Indiana University, Bloomington, Indiana 47405 USA
Received for publication May 11, 1999. Accepted for publication February 10, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: adaptive plasticity • competition • environmental cues • photoperiod • population differentiation • shade-avoidance responses • temperature • R:FR
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Harvell, 1990, 1998
; Schmitt, McCormac, and Smith, 1995
; Dudley and Schmitt, 1996
; Weinig, 2000a
). In plants, light quality is reliably correlated with the presence of neighbors and mediates competitive elongation responses in advance of direct shading (Casal and Smith, 1989
; Ballaré, 1990; Schmitt and Wulff, 1993
; Schmitt, 1997). However, multiple environmental factors may vary simultaneously and limit adaptive responses to a single cue such as light quality. Current temperature or photoperiod may affect elongation responses because both abiotic factors influence growth. Past conditions may also determine the expression of adaptive phenotypes because the maternal environment affects offspring traits (e.g., Alexander and Wulff, 1985
; Roach and Wulff, 1985
; Lacey, 1991
; Wulff and Bazzaz, 1992
; Fox and Savalli, 1998
). An individual's competitive ability may therefore depend not only on sensitivity to cues regarding neighbor proximity but also on the maternal environment and ambient temperature and photoperiod.
Neighbor proximity and the onset of competition for light are associated with shifts in the ratio of red and far-red wavelengths (R:FR) (Smith, 1982
; Casal and Smith, 1989
; Ballaré, 1990; Schmitt and Wulff, 1993
; Schmitt, 1997; Smith and Whitelam, 1997
). Light reflected off plants has a lower R:FR than sunlight because chlorophyll disproportionately absorbs light in the red region of the spectrum (Kasperbauer, 1971
; Holmes and Smith, 1977a, b
). Changes in incident R:FR are detected by the family of phytochrome photoreceptors, which switch reversibly between active and inactive forms in response to these light spectra (reviewed in Quail, 1991
; Furuya, 1993
). As the ratio decreases, the proportion of total phytochrome in the active form is altered (Smith and Holmes, 1977
), and stem elongation is increased relative to plants experiencing full-spectrum light (Morgan and Smith, 1976, 1978
; Lecharny and Jacques, 1980
; Child and Smith, 1987
). Under crowded conditions, individuals with greater stem elongation have higher light interception, accumulate more biomass, and achieve higher fitness than shorter individuals (Schmitt, McCormac, and Smith, 1995
; Dudley and Schmitt, 1996
; Weinig, 2000a
). Selection to increase or decrease elongation also depends on competitor identity, because interspecific differences in growth form alter the timing of competition for sunlight (Weinig, 2000a)
.
In addition to neighbor presence, an individual's level of elongation may depend on temperature or photoperiod. Both factors are likely to differ between seed cohorts that germinate relatively early or late and are known to influence levels of elongation (Myster and Moe, 1995
; Jensen et al., 1996
; Myster et al., 1997
; Neilly, Hickleton, and Kristie, 1997
; Talon and Zeevart, 1992
). This study addresses the following questions: (1) Does the maternal environment influence seedling elongation responses to R:FR? (2) Do seedling elongation responses to R:FR vary depending on ambient temperature or photoperiod? (3) Are populations that typically experience differences in temperature and photoperiod conditions at germination genetically differentiated such that responsiveness to R:FR depends on background temperature and photoperiod conditions? A common-garden growth chamber study using an annual weed, Abutilon theophrasti (Malvaceae), or velvetleaf, was performed to test these questions. Elongation responses of seedlings to R:FR were compared between individuals raised either under comparatively long and short photoperiods or high and low temperatures.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). It has subsequently become an aggressive invader of crop fields (Munger et al., 1987
; Warwick and Black, 1985
; Regnier and Stoller, 1989
; Patterson, 1992
).
Abutilon theophrasti is particularly suited to study the effects of temperature and photoperiod on responsiveness to light quality, because populations consistently experience differences in the two abiotic factors and in selection for elongation. Populations naturally occur in both disturbed, weedy areas and cultivated fields. Because of cultivation practices in the midwest, individuals occurring in weedy areas emerge prior to those in cultivated fields; seedlings in weedy areas emerge as soon as abiotic conditions permit (potentially early March), while seedlings in cultivated fields emerge later (early May) because crop planting is delayed to avoid frost damage. In response to constant growth conditions, populations may have become genetically differentiated for growth under specific abiotic conditions. Such differentiation is possible because interpopulation gene flow is likely limited. Seed dispersal distances should be low, because seeds are passively dispersed from dehiscing capsules. Pollen movement should also be minimal, because self-fertilization of seeds approaches 97% in natural populations (Andersen, 1988
). Any effects of temperature and photoperiod on responsiveness to light quality are particularly relevant to individual fitness in this species, because selection strongly favors enhanced seedling elongation in some sites (Weinig, 2000a)
.
Seeds used in this study were collected from two populations in each of the two site types: disturbed, weedy areas and fields undergoing continuous corn cultivation. Populations in the weedy site type were located at the Indiana University Faculty-Student Gardens and at the Indiana University aviaries in Bloomington, Indiana. Populations from fields undergoing continuous corn cultivation were located in Kokomo, Indiana and in Rosemont, Michigan. Both fields had been in cultivation between 15 and 20 yr. All populations used here were widely separated from one another and from populations in alternative site types. A previous study found that these populations were genetically differentiated for seedling elongation such that the populations from the cornfields exhibited greater elongation relative to populations from weedy sites (Weinig, 2000b)
. Seeds were collected from all populations in 1996. Seeds used in this study include both field-collected seed and the selfed progeny of 60 individuals (15 individuals from each population) raised for one generation in the greenhouse.
Experimental design
Two experiments were performed to examine the effects of temperature, photoperiod, and maternal environment on plastic elongation responses to light quality. In the first experiment, a fully factorial design of population x temperature x light quality was used. Individuals from each of the four populations listed above were raised under two separate temperature regimes. Within each temperature treatment, individuals were exposed to one of two light-quality conditions, spectrally normal light or simulated foliar shade. In a second experiment, individuals from the same four populations were raised under short and long photoperiods. Within each photoperiod treatment, individuals were again assigned to one of the two light-quality treatments. Seeds of plants grown in the greenhouse were used in both the temperature and photoperiod experiments. To directly assess maternal environmental contribution to elongation, growth responses of greenhouse-grown seeds were compared with those of field-collected seeds under the shorter photoperiod treatment.
Experiments examining elongation were performed in a Conviron environmental chamber (Conviron Controlled Environments Limited, Winnipeg, MB. Canada) at Indiana University. The chamber was illuminated using General Electric Cool White and Grolux VHO fluorescent tubes and 60-W inflorescent bulbs. This combination results in a R:FR ratio of 1.3, only slightly higher than ratios observed under full-sun conditions (Smith, 1982
). Foliar-shade conditions were simulated through the use of vinyl panels painted with pigments that reduce red-light transmittance (Lee, 1985
; Dudley and Schmitt, 1995
; Weinig, 2000b
). A 3.5% suspension of Hostperm Violet RL pigment (American Hoechst Inc., Coventry, Rhode Island, USA) in clear finish varnish was sprayed onto 8-gauge clear vinyl panels. Exact spectral shifts resulting from this pigment mixture were assessed using a LI-1800 portable spectroradiometer (LICOR Inc., Lincoln, Nebraska, USA). The panels reduced R:FR to between 0.4 and 0.45, which closely resembles light quality under natural canopies (Smith, 1982
). Photosynthetically active radiation (PAR) was 145 µmols·m-2·s-1 under the foliar-shade conditions and 330 µmols·m-2·s-1 in the full-spectrum treatment. Differences in elongation (see Results below) are attributed to light quality rather than light quantity, because many studies have established that the dramatic increases in stem elongation observed in crowded relative to uncrowded individuals are controlled by light quality rather than light quantity (reviewed in Schmitt and Wulff, 1993
; reviewed in Smith, Casal, and Jackson, 1990
; Dudley and Schmitt, 1995
). Among A. theophrasti exposed to experimental light treatments for 1 wk, light quality strongly affected elongation (i.e., greater than twofold increases in elongation between low and high R:FR groups of equivalent PAR), while light quantity had no effect (i.e., nonsignificant differences in elongation between high- and low-PAR groups) (Weinig, 2000b)
.
The experiment examining the effects of temperature was performed in two parts starting on 3 May and 15 May 1997. On the first date, 56 seeds from each population were soaked in 60°C water for 10 min in order to reduce population differences in timing of germination. Fourteen seeds of each population were then sown at uniform depth (1 mm) in one of four, 196-cell trays filled with Metro-mix 360 soil mixture, after which trays were placed in the growth chamber. The chamber was programmed for 18°/16°C day/night temperatures and 11-h photoperiods. On 15 May 1997, a second group of seeds was similarly soaked in water and planted. For this set of seeds, the Conviron was set for 26°/20°C day/night temperatures and 11-h photoperiods. The low- and high-temperature treatments reflect average daily temperatures in central Indiana in early April and early May, respectively (NOAA Climate Diagnostics Center, 1999). The use of lower temperatures, which would have more closely resembled conditions in early March, was not possible due to temperature limits of the Conviron. After each planting, seeds were checked every 12 h for germination. Germination began 2 d postplanting and was scored as the date of hypocotyl emergence. Three days after germination began, two trays were randomly selected for the foliar-shade treatment and two for the full-light treatment. The experiment was blocked such that a foliar-shade and a full-light treatment group were set up in each end of the chamber. A census of hypocotyl length was taken when the seedlings were 6 d old. Differences among populations and treatments in hypocotyl length were used to estimate the influence of these main effects on elongation, because differences in length should result from underlying differences in elongation.
The photoperiod experiment was similarly performed in two parts in the Conviron controlled-environmental chamber. On 16 and 30 October 1997, two sets of seeds were soaked, sown, and monitored for germination as above. Seeds in the first treatment group were raised under short, 11-h days, while seeds in the second group were placed under 14-h days. Eleven- and 14-h days resemble photoperiods occurring in early March and early May in Bloomington, Indiana (Astronomical Applications—U.S. Naval Observatory, 1999
). Both groups experienced day/night temperatures of 25°/20°C. As in the temperature experiment, seedlings were placed under the light-quality treatments 3 d postgermination, and hypocotyl length was censused when the seedlings were 6 d of age. One week later, hypocotyl length was again measured as was first internode length. When the seedlings were 13 d old, they were harvested. Leaf and stem tissues were collected, dried for at least 4 d at 60°C, and weighed.
Replicates of individual temperature and photoperiod treatments were carried out to ensure that photoperiod and temperature effects on elongation resulted from differences in these abiotic factors rather than heterogeneity between Conviron experiments.
Data analysis
All data were analyzed using the GLM procedure in SPSS 8.0 (Norusis, 1997). For the temperature experiment, data were pooled across blocks because the three-way interaction of block x light treatment x population was nonsignificant in both the low- and high-temperature treatments (F = 1.06, df = 3, 164, P = 0.37 and F = 1.38, df = 3, 144, P = 0.25, respectively). Data were similarly pooled across blocks in the photoperiod experiment because the three-way interaction of block x light treatment x population was nonsignificant in both short and long photoperiods (F = 0.33, df = 3, 181, P = 0.80 and F = 2.00, df = 3, 154, P = 0.12, respectively). To determine population and treatment effects on elongation and biomass accumulation, four-way nested ANOVAs were performed with site type (i.e., cornfields or weedy areas), light quality, and photoperiod or temperature as fixed main effects and with population nested within site type as a random effect. A separate set of four-way ANOVAs were conducted to assess the relative contribution of maternal environment to elongation. In these analyses, site type, seed source (field- vs. greenhouse-grown parents) and light quality were entered as fixed effects and population nested within site type as a random effect. Error terms appropriate for hypothesis tests in mixed models were determined from the SPSS GLM procedure for all analyses (Norusis, 1997). Germination date and seed mass were included as covariates when significant at P < 0.15.
In these experiments, an effect of temperature or photoperiod on plastic, shade-avoidance ability would be detected as an interaction between an abiotic factor (either temperature or photoperiod) and light-quality conditions. Population differentiation for elongation responses to temperature or photoperiod conditions would be detected as a significant abiotic factor x population interaction. A significant three-way interaction would show that the responsiveness of a population to foliar shading depends on temperature or photoperiod conditions.
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RESULTS |
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Maternal effects
Seedling elongation was compared between individuals derived from field- and greenhouse-collected seed. Seed source did not significantly affect hypocotyl elongation, indicating that maternal environment is unlikely to affect an individual's shade-avoidance phenotype (Table 3).
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DISCUSSION |
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; Lloyd, 1984
; Schlichting, 1986
; Sultan, 1987
; Watson, Geber, and Jones, 1995
). However, the physical environment is complex, and the expressed phenotype reflects an integrated response to many factors (e.g., Parrish and Bazzaz, 1985
; Benner and Bazzaz, 1987, 1988
; Casal and Smith, 1988a, b(??), 1989
; Miao, Bazzaz, and Primack, 1991
; Parks, Quail, and Hangarter, 1996). The data presented here indicate that temperature and photoperiod alter elongation responses to R:FR, illustrating that responses to light quality are more variable than previously realized. In addition, population responsiveness to R:FR varied with both abiotic factors. In natural populations, current temperature and photoperiod may affect the outcome of competitive interactions by limiting the expression of adaptive, elongated phenotypes.
Proximate mechanisms and their effects on competitive ability
Both average elongation and responsiveness of elongation to light quality were elevated in the high- relative to low-temperature treatments. The observed differences in stem length probably reflect the direct effects of temperature on cell elongation. Positive day-night temperature differences (i.e., day temperature minus night temperature) can dramatically increase cell elongation in stems (Erwin, Velgruth, and Heins, 1994
; Myster and Moe, 1995
; Jensen et al., 1996
; Myster et al., 1997
; Neilly et al., 1997
). Elevated levels of cell elongation could also explain why responsiveness to R:FR is greater at higher temperatures. Alternatively, higher temperature could have affected elongation indirectly, by elevating whole-plant growth rates and biomass accumulation (e.g., Parrish and Bazzaz, 1985
; Coleman et al., 1991; Ackerly et al., 1992
; Patterson, 1992
; McLachlan, et al., 1993
). Larger plants might simply have longer stems. Such indirect effects of growth seem less likely, however, because the greater carbon resources and growth rate of individuals experiencing long vs. short photoperiods failed to enhance elongation responses to low R:FR. This is shown by the nonsignificant interaction effect of light quality x photoperiod on elongation (Table 2).
The observed temperature sensitivity of A. theophrasti should be relevant in determining the outcome of interspecific competitive interactions, regardless of the underlying physiological mechanism. In competitive, weedy sites, seedling elongation is strongly correlated with fitness (Weinig, 2000a)
. Individuals in such sites may be more successful in interspecific competitive interactions at high relative to low prevailing temperatures (see also, Parrish and Bazzaz, 1985
; Patterson, 1992
), because responses to competitive cues will be enhanced. Any advantage conveyed by temperature sensitivity depends, of course, on the relative temperature sensitivity of interspecific competitors.
Photoperiod affected average elongation in full-spectrum light but not responsiveness to light-quality cues. Competitive ability of seedlings is therefore unlikely to be affected by photoperiod, because individuals should remain responsive to the cues associated with neighbors. Competitive success at later life-history stages may vary with daylength, depending on the mechanism underlying sensitivity of average elongation to photoperiod. Growing shoots can detect and respond to the current photoperiod (Talon and Zeevart, 1992
). If internode responses observed here reflect such direct responses to photoperiod, then elongation responses should be independent of photoperiod at both seedling and later life-history stages. However, the observed effects of photoperiod on seedling elongation may result from flowering induction, which both augments early growth and curtails later elongation in many species including A. theophrasti (Gilmour et al., 1986
; Juntilla and Jensen, 1988
; Talon and Zeevart, 1992
; Olsen, Juntilla, and Moritz, 1995
; Patterson, 1995
). Abutilon theophrasti enters an inductive, developmental phase, in which photoperiod affects flowering timing, 5 d postemergence (Patterson, 1995
). If flowering induction explains the observed differences in internode lengths, competitive success at later life-history stages may be diminished under long photoperiods as a result of diminished elongation. The effects of photoperiod on reproductive timing and elongation could explain why competitive interactions between A. theophrasti and soy (Glycine max) vary with planting date (Oliver, 1979
). The detrimental influence of the weedy species on soy growth is more pronounced when the crops are planted in early May than in July, potentially because seedlings of A. theophrasti that emerge earlier in the season prolong vegetative growth and attain greater heights.
Maternal environment did not influence seedling elongation. This result is consistent with patterns of phytochrome localization. Of the five known phytochromes (A–E), phytochrome A is present in large concentrations in seeds and in dark-grown tissues (Sharrock and Quail, 1989
; Smith and Whitelam, 1990
; Quail, 1991
). Levels of this phytochrome decrease rapidly in seedlings experiencing sunlight (Komeda et al., 1991
; Adam et al., 1997
; Quail, 1991
; Somers and Quail, 1995
). A consequence of this variability in phytochrome A concentration is that the seedling light environment and seedling phytochromes should influence elongation more strongly than the maternal environment, although the latter determines the initial complement of phytochrome in seeds.
Population differences and their evolutionary implications
Interpopulation differences in sensitivity of elongation to temperature may influence allocation patterns and fitness under noncompetitive conditions. Elongated plants perform less well than shorter ones under noncompetitive conditions, because they experience the cost of diverting carbon resources from leaves to stems and fail to derive a carbon return from this allocation strategy (Schmitt, McCormac, and Smith, 1995
; Dudley and Schmitt, 1996
; Weinig, 2000a
). Elongated plants also divert resources from root production (Maliakal et al., 1998; Cipollini and Schultz, 1999
). In the high-temperature treatment, plants from the cornfield populations were more elongated than those from the weedy populations (Fig. 2). Individuals from the cornfield populations may therefore experience reduced fitness relative to individuals from the weedy populations under hot, noncompetitive conditions because resources will be allocated preferentially to stems rather than resource-acquiring organs (see also Coleman and Bazzaz, 1992
).
For individuals derived from the experimental cornfield populations, the outcome of competitive interactions may be affected by photoperiod. Relative to individuals of the second cornfield population, individuals from the first cornfield population elongated more in response to light-quality cues when growing under a short photoperiod (Fig. 5a). These population rankings for responsiveness to light-quality cues were reversed under the longer photoperiod (Fig. 5b). This reversal suggests that photoperiod may determine the relative success of individuals from each cornfield population in competitive sites, because seedling elongation affects fitness in many of the environments that A. theophrasti normally inhabits (Weinig, 2000a)
.
The results of this study and others (Sultan, 1993a
, b, c) illustrate that multiple environmental factors can affect a single aspect of the phenotype. Evaluating the adaptive benefits of plastic responses therefore depends on an understanding of how environmental factors interact to affect development and the expression of phenotypic characters. The morphogenic effects of temperature and photoperiod observed in this study are common to many species (Karlsson and Heins, 1986; Bertram and Karlsen, 1994
; Erwin, Velgruth, and Heins, 1994
; Myster and Moe, 1997
). As a result, temperature and photoperiod conditions may be relevant in determining competitive dynamics in many natural plant populations.
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FOOTNOTES |
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2 Current address: Department of Ecology and Evolutionary Biology, Box G-W, Brown University, Providence, Rhode Island 02912 USA. (Tel: 401-863-2897; FAX: 401-863-2166; e-mail: cweinig{at}brown.edu
).
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